Method and apparatus for a high resolution downhole spectrometer

ABSTRACT

The present invention provides a simple, robust, and versatile high-resolution spectrometer that is suitable for downhole use. The present invention provides a method and apparatus incorporating a spinning, oscillating or stepping optical interference filter to change the angle at which light passes through the filters after passing through a sample under analysis downhole. As each filter is tilted, the color or wavelength of light passed by the filter changes. Black plates are placed between the filters to isolate each filter&#39;s photodiode. The spectrometer of the present invention is suitable for use with a wire line formation tester, such as the Baker Atlas Reservation Characterization Instrument to provide supplemental analysis and monitoring of sample clean up. The present invention is also suitable for deployment in a monitoring while drilling environment. The present invention provides a high resolution spectometer which enables quantification of a crude oil&#39;s percentage of aromatics, olefins, and saturates to estimate a sample&#39;s gas oil ratio (GOR). Gases such as CO 2  are also detectable. The percentage of oil-based mud filtrate contamination in a crude oil sample can be estimated with the present invention by using a suitable training set and chemometrics, a neural network, or other type of correlation method.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. patent application No. notpresently assigned, entitled “A Method and Apparatus for a DownholeFlourescence Spectrometer” by Rocco DiFoggio, Paul Bergen and ArnoldWalkow, filed on Jun. 4, 2002 which is hereby incorporated herein byreference in its entirety. This application is related to U.S. patentapplication No. not presently assigned, entitled “A Method and Apparatusfor a Derivative Spectrometer” by Rocco DiFoggio, Paul Bergen and ArnoldWalkow, filed on Jun. 4, 2002 which is hereby incorporated herein byreference in its entirety. This application is related to the U.S.patent application Ser. No. 10/119,492 filed on Apr. 10, 2002 by RoccoDiFoggio et al., entitled “A Method and Apparatus for DownholeRefractometer And Attenuated Reflectance Spectrometer” which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention pertains in general to a high-resolution downholescanning spectrometer that is suitable for downhole use and inparticular, to a downhole spectrometer that employs spinning,oscillating, or stepping one or more optical interference filters tochange the angle at which light passes through them to obtain a higherresolution measurement.

[0004] 2. Summary of the Related Art

[0005] Oil companies take samples from formations to determine thecharacteristics of the formation. These samples are typically pumpedfrom the formation and initially contain contaminants such as well borefluid that have invaded the formation. Contaminated sample yield invalidresults when trying to determine the properties of a hydrocarbon bearingformation. Thus, oil companies desire an accurate measure of samplecontamination percentage in real time as they are pumping sample fluidfrom a formation so that they can decide when to divert a reasonablypure formation fluid sample into a collection tank. They do not want topump unnecessarily long and waste very expensive rig time. Conversely,they do not want to pump too little and collect a useless sample. If thecontamination is more than about 10%, the sample may be useless.

[0006] In such cases, the PVT properties measured in the lab cannot becorrected back to true reservoir conditions because of this excessivecontamination. It is therefore necessary to perform measurementsdownhole to assess the sample contamination and associated merits ofinformation regarding formation properties derived from the downholesample. One method of investigation comprises using a spectrometer toperform optical measurements on the fluid samples as they are pumpedthrough a sampling instrument and subsequently collected in a downholeenvironment.

[0007] Numerous factors affect downhole spectrometry measurements. Inthe down-hole environment, photodetectors are utilized and must operateat high ambient temperatures, thus, they are very noisy and generatevery little signal. Moreover, dirty or contaminated samples of flowingstreams of crude oil containing scatterers such as sand particles or gasbubbles tend to add noise to the system. These scatterers cause theoptical spectrum to momentarily “jump” up, appearing darker as thescatterers pass through the sample cell. At high concentrations, thesescatterers cause the measured spectrum to jump or rise up repeatedly. Tofirst order, the scattering effect is just a momentary baseline offset.One way to eliminate baseline offset and greatly improve thesignal-to-noise ratio of a downhole spectrometer is to collect thederivative of the spectra with respect to wavelength. Derivative spectracan be obtained by modulating the wavelength of light and using alock-in amplifier.

[0008] It is commonplace for spectrometers to disperse white light intoits constituent colors. The resulting rainbow of colors can be projectedthrough a sample and onto a fixed array of photodetectors.Alternatively, by rotating a dispersive element (i.e. grating, prism),the rainbow can be mechanically scanned past a single photodetector onecolor at a time. In either case, an operator can obtain a sample'sdarkness versus wavelength, in other words, its spectrum.

[0009] Photodetectors and their amplifiers almost always have somethermal noise and drift, which limit the accuracy of a spectral reading.As operating temperature increases, noise and drift increasedramatically at the same time that photodetector signal becomessignificantly weaker. If an operator oscillates the wavelength (color)of light about some center wavelength, then the operator can reject mostphotodetector and amplifier noise and drift by using an electronicbandpass filter, centered at the oscillation frequency. The operator canfurther reject noise by using a phase-sensitive (“lock-in”) amplifierthat not only rejects signals having the wrong frequency but alsorejects signals having the correct frequency but having no fixed phaserelationship (indicative of noise) relative to the wavelengthoscillation. A lock-in amplifier can improve signal to noise by as muchas 100 db, which is a factor of 10^(100 db/10) or 10 billion.

[0010] The output of the lock-in amplifier used in this procedure isproportional to the root-mean-square (RMS) amplitude of that portion ofthe total signal, that portion being at the same frequency and having afixed phase relationship relative to the optical frequency beingobserved. The more that the darkness of the sample changes with color,the larger importance this RMS value will have to the operator. In otherwords, the output of lock-in amplifier for a system with anoscillating-wavelength input is proportional to the derivative of thespectrum (with respect to wavelength) at the center wavelength of theoscillation.

[0011] U.S. Pat. No. 3,877,818, Photo-Optical Method for Determining FatContent in Meat, issued Apr. 15, 1975, to Button et al. discloses anapparatus in which light reflected off the surface of a piece of meatpasses through a lens and strikes an oscillating mirror. The angle ofreflection of the light reflected off this mirror varies with themirror's oscillation. Light reflected from the mirror strikes astationary interference filter and onto a photodetector. The colortransmitted through the interference filter varies slightly with theangle of incidence of the light beam striking it. Thus, the wavelengthof radiation passing through the filter oscillates over a narrow rangeabout the wavelength for fat absorption of meat and can be used todetermine the fat content of the meat. Button et al. does not enable thepossibility of a full rotation or control of the angular deviation.

[0012] Current down-hole “spectrometers” (for example, Schlumberger'sOptical Fluid Analyzer (OFA) and Baker Atlas' SampleView^(SM)) utilizediscrete filter photometers. Each optical channel is achieved byfiltering light for each individual channel through an optical filter ofa color corresponding to the optical channel. Therefore, the wavelengthcoverage of such a discrete spectrometer is not continuous, butdiscrete. The discrete spectrum has gaps that go from the centerwavelength of one discrete optical filter to the center wavelength ofthe next discrete optical filter. These gaps can be large ones of 100 to200 nm or more in wavelength coverage. The channels of such devices arebroad. In the current down-hole spectrometer, all hydrocarbon peaks arelumped into one broad channel centered at 1740 nm, with a Full Width atHalf Maximum (FWHM) of 32 nm.

[0013] The down-hole environment is difficult for operating sensors.Reasons include limited space within a tool's pressure housing, elevatedtemperatures, and the need to withstand shock and vibration. Componentssuch as motors, interference filters, and photodiodes in the loggingtools, are already fabricated and available to withstand temperature,shock and vibration of the downhole environment. Thus, it is possible tomanufacture this spectrometer into a small enough package in order tosqueeze into the available space inside the current SampleView™ module.

[0014] For comparison, many laboratory spectrometers use a grating todisperse the light into its constituent colors. However, almost allgratings are an epoxy-on-glass replica of a master grating. These epoxyreplicas soften and creep at high temperature, which causes a distortionin the spectrum and a loss in light intensity. Moreover, the price of anall-glass master grating ($50-100K) is prohibitively expensive.

[0015] Known laboratory spectrometers typically utilize FourierTransform (FT) spectroscopy which are unsuitable for down-holeapplications. FT spectrometers are large (2-3 feet long, 1-2 feet wide),heavy (200-300 lbs), mechanically and electronically complicated, andmust maintain perfect alignment of all their optical components to workproperly, which is why they are typically built on a very rigidframework. Thus, there is a need for a high-resolution spectrometerwhich is small enough and robust for operation in a downholeenvironment.

SUMMARY OF THE INVENTION

[0016] The present invention provides a simple, robust, and versatilehigh-resolution spectrometer that is suitable for downhole use. Thepresent invention provides a method and apparatus incorporating aspinning, oscillating or stepping optical interference filter to changethe angle at which light passes through the filters after passingthrough a sample under analysis downhole. As each filter is tilted, thecolor or wavelength of light passed by the filter changes. Black platesare placed between the filters to isolate each filter's photodiode. Thespectrometer of the present invention is suitable for use with a wireline formation tester, such as the Baker Atlas ReservationCharacterization Instrument to provide supplemental analysis andmonitoring of sample clean up. The present invention is also suitablefor deployment in a monitoring while drilling environment. The presentinvention provides a high resolution spectometer which enablesquantification of a crude oil's percentage of aromatics, olefins, andsaturates to estimate a sample's gas oil ratio (GOR). Gases such as CO₂are also detectable. The percentage of oil-based mud filtratecontamination in a crude oil sample can be estimated with the presentinvention by using a suitable training set and chemometrics, a neuralnetwork, or other type of correlation method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an illustration of the present invention deployed in adownhole environment in a bore hole;

[0018]FIG. 2 is a schematic representation of a preferred embodiment ofthe present invention;

[0019]FIGS. 3A and 3B illustrate a more detailed schematicrepresentation of a preferred embodiment of the present inventionshowing a time groove;

[0020]FIG. 4 illustrates an oscilloscope display for an empty 2 mm pathlength glass sample cell;

[0021]FIG. 5 illustrates an oscilloscope display for a 2 mm glass samplecell full of water;

[0022]FIG. 6 illustrates an oscilloscope display for a 2 mm glass samplecell full of crude oil;

[0023]FIG. 7 illustrates a processed spectrum of the same crude oiltaken on a research-grade Cary 500 laboratory spectrometer;

[0024]FIG. 8 illustrates how a Full Width at Half Maximum of thetransmission of an 1800 nm filter changes with tilt;

[0025]FIG. 9 shows the correlation (r²=0.993) of filtrate contaminationto two wavelengths of a training set of forty-three2-wavenumber-resolution spectra;

[0026]FIG. 10 shows the correlation (r²=0.908) of filtrate contaminationto two wavelengths of a training set of forty-three22-wavenumber-resolution spectra; and

[0027]FIG. 11 compares the near infrared absorption bands of gaseouscarbon dioxide with the near infrared absorption bands of water vapor.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0028]FIG. 1 illustrates a preferred embodiment of the present inventiondeployed in a borehole. The present invention is suitable for deploymentin either a wire line, slick line or monitoring while drillingenvironment. FIG. 1 illustrates a preferred embodiment of the presentinvention deployed in a monitoring while drilling operation.

[0029] Turning now to FIG. 1, FIG. 1 is a drilling apparatus accordingto one embodiment of the present invention. A typical drilling rig 202with a borehole 204 extending there from is illustrated, as is wellunderstood by those of ordinary skill in the art. The drilling rig 202has a work string 206, which in the embodiment shown is a drill string.The drill string 206 has attached thereto a drill bit 208 for drillingthe borehole 204. The present invention is also useful in other types ofwork strings, and it is useful with a wireline, jointed tubing, coiledtubing, or other small diameter work string such as snubbing pipe. Thedrilling rig 202 is shown positioned on a drilling ship 222 with a riser224 extending from the drilling ship 222 to the sea floor 220. However,any drilling rig configuration such as a land-based rig may be adaptedto implement the present invention.

[0030] If applicable, the drill string 206 can have a downhole drillmotor 210. Incorporated in the drill string 206 above the drill bit 208is a typical testing unit, which can have at least one sensor 214 tosense downhole characteristics of the borehole, the bit, and thereservoir, with such sensors being well known in the art. A usefulapplication of the sensor 214 is to determine direction, azimuth andorientation of the drill string 206 using an accelerometer or similarsensor. The bottom hole assembly (BHA) also contains the formation testapparatus 216 of the present invention, which will be described ingreater detail hereinafter. A telemetry system 212 is located in asuitable location on the work string 206 such as above the testapparatus 216. The telemetry system 212 is used for command and datacommunication between the surface and the test apparatus 216.

[0031]FIG. 2 illustrates a preferred embodiment of the invention. Alight source 101 (e.g. a tungsten light bulb) provides a beam of light102 which is targeted to be incident upon and through a collimating lens103 onto a sample 105 which contains a sample to be analyzed by thepresent invention. In a preferred embodiment, formation fluid samplesflow through sample chamber (chamber not shown for simplicity) and aretested for purity or other properties by the present invention. Thecollimated light is shown upon and transmitted through the sample 105and towards a filter assembly. The filter assembly comprises one or moreoptical filters 109 positioned between baffles 111 and mounted on arotating spindle 107. FIG. 2 illustrates, for example, an assembly inwhich two optical filters 109 are positioned between three baffles 111.A rotating spindle 107 is oriented with its axis generally perpendicularto the direction of the light transmitted from the sample and rotatesabout its axis. The one or more optical filters 109 are mounted on thespindle so that, in one angular orientation of the spindle, the plane ofthe filters are generally perpendicular to the light transmitted fromthe sample. As the spindle rotates, light 106, after passing throughlens 103, impinges on the face of interference filters 109 at incidentangles other than normal. A processor 118 controls data acquisition andanalysis for the present invention.

[0032] Light transmitted by the interference filters 109 is transmittedto one or more photodetectors 113 (e.g., InGaAs photodetectors). Thefilters are specified according to their center wavelength fortransmitted light at normal incidence. However, their center wavelengthfor transmitted light is different at non-normal incidence. As one tiltsa single-color optical interference filter so that white light no longerstrikes the filter perpendicular to its face, the color (that is,wavelength) of the light that is transmitted, shifts from its originalcolor to a shorter wavelength. The greater the angle of tilt of aninterference filter away from normal incidence, the greater the shift inthe wavelength. For example, tilting a yellow (λ=590 mn) interferencefilter, the transmitted light shifts away from yellow. With sufficienttilt from normal, the light transmitted by the “yellow” filter shifts togreen (λ=525 mn). Interference filters are chosen according to desiredapplications. As an example, a λ=1800 nm wave length interferencefilter, when rotated, transmits over a broad range of wavelengths sothat the range of wavelengths scans past the hydrocarbon absorbancepeaks located around λ=1760 nm.

[0033] The external surfaces of the housing and the spacer disks andstray light barriers are optically black to prevent stray light frominterfering with the light to be measured. One exception is a narrowmachined timing groove 117 as shown in FIG. 3 and a second timing grooveon the opposite side of the housing to obtain two timing marks perrevolution and to select which filter is being read and from which sideit is read or alternatively to average the readings through both sides.Additional timing grooves are provided for added angular displacementresolution for rotational position determination. The light illuminatesthe timing groove when it appears, and a timing detector diode sensesthe light from the timing groove at which time the data acquisitionelectronics are alerted to obtain and analyze data.

[0034] The present invention provides for full exposure of eachoptically isolated filter during rotation around an exposed center ofrotation. Each filter is held by its edges to avoid shadowing thefilter's center of rotation. The present invention enables full analysisand expanded capability to read only a designated side of each filter oran average of both sides. Four spectral scans per revolution areprovided, which are selectable so that any scan can be selectedseparately or two or more scans can be averaged.

[0035] For collimated light that is incident on an interference filter,the transmitted wavelength is given by the following formula,

λ=λ₀[1−(n ₀ n*)² sin² θ]^(1/2)   (1)

[0036] where λ is the central wavelength of the filter for lightincident at an angle θ from normal, λ₀ is the central wavelength of thefilter for light at normal incidence (θ=0), n₀ is the refractive indexof medium surrounding the filter (in our case, air so n₀=1), n* is theeffective refractive index of the filter, and θ is the angle from normalincidence (θ=ωt, where ω=angular velocity, t=time). The wavelength shiftof the filter depends only on the angle by which the incident light beamis rotated away from normal incidence, regardless of whether thatrotation is positive or negative. Thus, it is expected that oscilloscopedisplays are symmetrical (a mirror image) about the center of eachspectrum. This center spectrum corresponds to a filter's nominal centerwavelength, for example, λ=1800 nm. The further right or left of thecenter spectrum, the shorter the wavelength. Since the filter rotates onits spindle and at a constant rate so that angle depends on time, thenthe wavelength, λ, decreases with the absolute value of time, |t−t₀|,where t₀ is the moment in time at which the filter is centered on itscentral wavelength.

[0037] Typically, the range of shift for the wavelength of lighttransmitted from the filter is about 100 nm. As the angle of rotationincreases, the filter becomes so tilted that almost no light goesthrough the face of the filter. Instead, most of the light getsreflected off the face, because it is striking it at such a glancingangle. At a large enough rotation angle, light runs into the edge of thefilter or into the edge of the filter's holder.

[0038] In one mode of the invention, by spinning or oscillating such aninterference filter, it is possible to quickly and continuously sweepthrough a range of wavelengths from the original center wavelength to anew center wavelength, which is about 100 nm shorter than the centerwavelength for a given filter. To increase the range of wavelengths thatare scanned by the present invention, the present invention preferablycomprises several strips of interference filters (having differentcentral wavelengths) side by side, but separated by thin black disks(baffles) 111. These interference filter strips rotate together as shownconceptually in FIG. 2. The thin black disks enable a photodiode placedbetween a pair of black disks to monitor the light transmitted by asingle filter without seeing light transmitted by a neighboring filter.

[0039] A more detailed depiction of a preferred embodiment is shown inFIG. 3. As shown in FIG. 3A, a motor and gear train assembly 110provides rotational force to spindle 107 which in turn rotates housing120 and filter plates 111. Stray light barriers 128 are provided betweenphotodetector diodes 113 to prevent leakage between individual diodes113 and associated channels of information. A timing groove 117 isilluminated by a lamp 123 to facilitate rotational and angular trackingof housing and filter 111. Light 106 emerging from sample to be analyzedis run through light tubes 127 to moving filter 111. A side view ofhousing 120 is shown in FIG. 3B.

[0040] In another mode of the invention, it is preferred to oscillatethe filter to scan over an absorption peak (e.g., λ=1667 nm for methane,or λ=1957 nm for carbon dioxide) in order to determine the difference intransmitted light intensity on-peak versus off-peak. This difference inon/off peak readings (or the amplitude of the resulting AC signal ifdone rapidly) is generally proportional to the concentration of theabsorbing species.

[0041] For light that is incident at an angle, which is not normal tothe filter, light still emerges from the filter parallel to the originalbeam (due to normal incidence). However, the path of the filtered lightis translation displaced (offset) from the path of the light emergingfrom the non-rotated filter. Therefore, a sufficiently large converginglens to collect the light, regardless of offset, enables the operationof the invention. Also, the optical bandpass of the filter changes withthe angle of incidence. The bandpass of the tilted filter is wider thanthe bandpass of the untilted filter. Note that color shifting with angleonly occurs for optical filters that are based on interference. Colorshifting does not occur for filters that are inherently colored.

[0042] The present invention enables the detection and analysis of smallpeaks or perturbations in the spectral response for a sample includingthe case of a small peak sitting on the shoulder of a larger peak. Forexample, the present invention may either step, oscillate or spin theinterference filter. In the case of a small perturbation or peak uponthe shoulder of another peak, the present invention may step between twocolors, one that is on the peak and one off of the peak. In this way, itcan detect small or overlapping peaks, which with previous downholespectroscopy techniques and equipment, were lumped into a single broadand undiscriminating hydrocarbon band. For example, in the absorbancespectrum, the methane peak is a side peak on the main hydrocarbon peak.The present invention enables detection of this small methane peak bystepping between the methane peak at 1667 nanometers and a lower offpeak color, for example 1600 nanometers. The present invention providesa stepper motor to step between positions or colors for the interferencefilter. By stepping between the on peak 1667 nm and off peak 1600 nm thepresent invention is able to detect the small peak at 1667 nm formethane. Alternatively, we could quantity the amount of methane byapplying chemometrics, neural networks, or other soft modelingtechniques to the continuous spectra obtained by the present inventionfor a known training set of samples.

[0043] The high-resolution spectrometer of the present invention enablesdetermination of the percent of aromatics, olefins, saturates andcontaminants in a sample. The high-resolution spectrometer of thepresent invention enables detection and analysis of peaks that wereheretofore undetectable with known downhole spectrometer technology.Known downhole spectrometers group together most first-overtonehydrocarbon peaks into a single hydrocarbon channel without resolvingthe small or overlapped peaks within this channel, which can provideinformation about concentrations of aromatics, olefins, saturates,filtrate contamination, and so on. In a preferred embodiment, thepresent invention collects high resolution spectra for a training set ofknown samples. Applying chemometrics to these spectra, we can developmathematical models, which allow us to estimate the correspondingproperties of unknown samples directly from their high resolutionspectra. Chemometrics, neural networks, and other soft modelingtechniques let us skip the intermediate steps of identifying chemicalmeanings for individual spectral peaks and the reasons that these peaksshift or overlap with other peaks. That is, soft modeling techniquesallow us to take high resolution spectra of a known training set anddirectly model the chemical or physical properties of interest. Oneproperty of interest is filtrate contamination. Known samplingtechniques do not currently provide a direct measurement of thepercentage of filtrate contamination in a formation fluid sample.

[0044]FIG. 9 plots a correlation (r²=0.993) found between the fractionof filtrate contamination and some very-high resolution (2-wavenumber)spectra of 3 crude oils (representing low, medium and high viscosity), 4synthetic mud filtrates, and 36 simulated mixtures of a crude oil with amud filtrate. These Attenuated Total Reflectance (ATR) spectra werecollected on a commercial laboratory spectrometer made by Nicolet. Thesespectra have higher resolution than we would be able to obtain with themethods of the present invention. They spanned the fundamentalfrequencies of molecular vibration.

[0045] Note, that with a single equation, we were able to quantify thepercentage of filtrate contamination regardless of which filtrate wascontaminating which crude oil. Apparently, underlying chemicaldifferences between filtrates and the crude oils can be detected andquantified. Here, the correlation is to a region around 3032wavenumbers. A wavenumber is one reciprocal centimeter. Therefore, 3032wavenumbers is 10⁷/3032=3298 nm because there are 10 million nanometersin a centimeter.

[0046] In FIG. 9, we see that the fraction of filtrate contamination isgiven by the equation:

f _(c)=−2.25−1072.71*A _(—)3037.38 cm⁻¹+1130.20*A _(—)3027.74 cm⁻¹

[0047] Because this correlation equation is based on the absorbance attwo wavelengths (3037.38 cm⁻¹ and 3027.74 cm⁻¹) that are close to oneanother and whose regression coefficients are approximately equal inmagnitude but opposite in sign (−1072.71 and 1130.20) it is, in essence,a correlation to the slope of the spectrum at the midpoint between thetwo wavelengths. It may be a correlation to the absence of aromatics inthe filtrates because, for environmental reasons, the base oils forthese synthetic muds are generally made without aromatics. The aromaticC-H stretch peak occurs from 3125-3030 cm⁻¹ whereas the non-aromatic C-Hstretch occurs around 2940-2855 cm⁻¹. Thus, it appears that the slope ofspectrum in the region where aromatic peaks end and non-aromatic peaksbegin is a way to directly measure the filtrate contaminationindependent of the filtrate, the crude, or any visible colors of either(this wavelength range is far past that of visible colors or anyelectronic transitions). We expect that overtones (somewhere in the1600-1800 nm range) of these fundamental bands (3125-2855 cm⁻¹) willhave corresponding sensitivity to the underlying chemical differencesbetween filtrates and crude oil.

[0048] In like manner, FIG. 10 plots the correlation (r²=0.908) betweenlower-resolution spectra, which were derived from the original spectraby processing them to reduce their resolution to 22 wavenumbers fullwidth half maximum. This processing was done to simulate the spectrathat might be obtained using a rotating interference filter, describedin the present invention, instead of a lab spectrometer. Note that, at3162 wavenumbers, the bandpass expressed in wavenumbers is exactly equalto the bandpass expressed in nanometers. At 3030 wavenumbers, a22-wavenumber bandpass represents approximately a 20-nm bandpass. Thepresent invention may also be used to detect CO2. As shown in FIG. 11,CO2 has bands of peaks near 1430 nm, 1575 nm, 1957 nm, 2007 nm, and 2057nmm. Some of these overlap with water or hydrocarbon or other absorptionpeaks. A technique such as that of the present invention, which allowsrapidly reading the on-peak versus off-peak absorbance may be able toresolve and quantify the peak regardless of such interferences.

[0049] FIGS. 4-7 illustrate an example of data taken with a spinningfilter apparatus in a laboratory. FIG. 4 shows an oscilloscope displayfor an empty 2 mm path length glass sample cell. The absorbance of theglass cell is both constant and negligible from 1700-1800 nm. However,the tungsten light source, which peaks in intensity around 1000 nm,continues to tail off in intensity in going from 1700 nm to 1800 nm.Indeed, the center wavelength (1800 nm) of the empty cell's raw spectrumis less than the intensity of wavelengths at either end (about 1700 nm)of the spectrum.

[0050]FIG. 5 shows an oscilloscope display for a 2 mm glass sample cellfull of water. Water has no major peaks between 1700 and 1800 nm.However, the region shown in FIG. 5 is at the tail end of a major waterpeak at 1930 nm. Consequently, water transmits less light at 1800 nmthan at 1700 nm. The oscilloscope display shows a raw spectrum similarto the empty cell display except for a deeper central dip due to waterbeing a little darker at 1800 nm than at 1700 nm.

[0051]FIG. 6 shows an oscilloscope display for a 2 mm glass sample cellfull of crude oil. Moving in either direction away from the center ofthe spectrum enables a user to see characteristic dips in thetransmitted light intensity (absorbance peaks) corresponding to thespectral signature of hydrocarbons. These absorbance peaks occur near1760 nm and 1725 nm.

[0052]FIG. 7 shows a processed spectrum of the same crude oil taken on aresearch-grade Cary 500 laboratory spectrometer. The processingcomprises correcting for the variation in the intensity of the lightsource at different wavelengths. A spectrum of the empty cell iscollected first, followed by the spectrum of the cell filled with asample. The base ten logarithm of the ratio of the empty cell lightintensity to the sample-filled cell light intensity is the “absorbance”of the sample in the cell and is plotted on the y-axis. It is preferableto generate processed rather than raw spectra from data collected usingthe apparatus of the invention.

[0053] Comparing FIGS. 6 to FIG. 7, the y-axis of each oscilloscopedisplay is a linear scale with light increasing in the up direction andwith no correction being made for variation in the intensity of thelight source with wavelength. However, the y-axis of the Cary spectra isa logarithmic scale with light decreasing in the up direction and acorrection being made for the variation in intensity of the light sourcewith wavelength. Conceptually, turning the oscilloscope display upsidedown and compressing the y-axis, produces a curve very similar to theCary spectra. The dips in the oscilloscope displays correspond to thepeaks in the Cary displays. Despite these y-axis dissimilarities, thewavelength positions of the peaks in the Cary displays can be used toassign wavelengths to the time scale of the oscilloscope displays. Also,we can assign wavelengths to the time scale by knowing the angle of therotating filter at each time (and the filter's normal-incidence centerwavelength). For simplicity, we can provide a motor, which spins thefilter at constant speed.

[0054] For a preferred embodiment of the high-resolution spectrometer,wavelength coverage is continuous and the wavelength step size isselected to be as small as possible to measure the angle of rotation.Using an interference filter having a sufficiently narrow bandpass, itis possible to perform a high-resolution scan over one or more portionsof the first overtone hydrocarbon band (1600-1850 nm) or over otherinteresting spectral features. These higher resolution spectra improveour ability to quantify chemical concentrations and to estimate anyspectrally correlated physical properties. For example, a user canquantify the percentages of aromatics, olefins, and saturates in thecrude oil and estimate the gas oil ratio (GOR) from the percentage ofmethane, whose peak at 1667 nm, lies left of heavier hydrocarbons peaksthat are around 1700-1800 nm.

[0055] The spectrometer of the present invention is used in conjunctionwith a wire line formation tester (e.g. the Reservoir CharacterizationInstrument (RCI)). It would supplement the existing 17-channel downholespectrometer, which currently monitors sample cleanup in real time.

[0056] With a high-resolution spectrometer, a user should be able toquantify a crude oil's percentage of aromatics, olefins, and saturates,to estimate its gas oil ratio (GOR). Gases such as CO₂ might bedetectable. With a proper training set, it is possible to develop acorrelation equation for the percentage of oil-based mud filtratecontamination in the crude oil.

[0057] Turning now to FIG. 8, FIG. 8 illustrates how the bandpass (FullWidth at Half Maximum of the transmission) of an 1800-nm filter changeswith tilt. Note that one-half of the transmission corresponds to a 0.3Absorbance increase because—log₁₀(½)=0.3. As shown in FIG. 8, thegreater the tilt angle, the more that the center wavelength shifts toshorter wavelengths and the broader the bandpass becomes about the newcenter wavelength. At 40 degrees from normal, FWHM=26 nm; at 20 degreesfrom normal, FWHM=16 nm; and at 0 degrees from normal, FWHM=11 nm. Asshown in FIG. 8, the resolution bandwidth is proportional to the angulardisplacement of the interference filter with respect to the normalincidence of light from the sample.

[0058] The foregoing example of a preferred embodiment is forillustration purposes only and not intended to limit the scope of thepresent invention which is defined by the following claims.

What is claimed is:
 1. A downhole spectrometer comprising: A sonde fortraversing a borehole; An instrument body in the sonde comprising: Atleast one interference filter attached to a spindle so that, uponrotation, the angle at which light strikes the filter changes; and atleast one light-detecting devices, with each light-detecting devicespaired with one interference filter; a collimated light beam incident ona fluid sample, a portion of light incident upon the fluid sample beingtransmitted through the sample, the transmitted light incident upon theface of at least one interference filter, with the light transmittedfrom the interference filter being incident upon the light-detectingdevices with which it is paired; and A rotator component to rotate theangle of the spindle and its attached the interference filters relativeto to the incident light upon the interference filters.
 2. The apparatusof claim 1, further comprising a light gathering lens for improvedsignal strength.
 3. The apparatus of claim 1, wherein the rotatorfurther comprises a stepper motor or DC servo motor or an encoder wheelto obtain the angle of rotation.
 4. The apparatus of claim 1, furthercomprising at least one baffle for optically isolating a filter-detectorpair from a neighboring filter detector pair, while rotating the entireassembly.
 5. The apparatus of claim 1, further comprising a converginglens that is large enough to capture the light beam exiting theinterference filter despite beam offset associated with tilting thefilter.
 6. The apparatus of claim 1, further comprising a holder forsupporting the at least one interference filter during rotation.
 7. Theapparatus of claim 1, further comprising said spectrometer capable ofwithstanding 175 C in operation, a vibration of 5 g and/or shock of 20 gduring shipping.
 8. A method of measuring the spectrum of a fluid samplein a downhole environment comprising: lowering a sonde containing asampling spectrometer into a bore hole; illuminating a fluid sample inthe sampling spectrometer; directing light from the fluid sample onto atleast one interference filter; directing light from at least oneinterference filter onto the fluid sample; rotating the one or moreinterference filters to obtain different wavelengths of lighttransmitted through the filters as the angle of incidence of the lightupon the rotating filter changes; recording the intensitiescorresponding to those wavelengths; and correlating angular position ofthe filter with measured light intensity to obtain a measure of thespectrum of the fluid sample at different wave lengths of light.
 9. Themethod of claim 8, further comprising: deriving the wavelength of lightfrom the filter's angle of rotation.
 10. The method of claim 8, furthercomprising: scanning a sample's spectrum by measuring at each angle ofrotation at least one of the light transmitted through the sample andthe light reflected off of the sample.
 11. The method of claim 8,further comprising: scanning four spectral scans with every 3600 offilter rotation at ±θ and at ±(θ+180) degrees.
 12. The method of claim11, further comprising: gating the four spectral scans intervals in timeor in angle so as to select a single scan or to average two or morescans.
 13. The method of claim 8, further comprising: rotating thefilter assembly freely on its spindle.
 14. The method of claim 8,further comprising: angularly oscillating the filter assembly about acentral angle.
 15. The method of claim 8, further comprising: steppingan angular rotation of the filter between two wavelength positions inorder to obtain an on-peak versus off-peak measurement.
 16. The methodof claim 8, further comprising: holding the filter by its edge so thatthe holder does not create a shadow at the filter's center of rotation.17. The method of claim 8, further comprising Determining a gas oilratio by measuring the percentage of methane.
 18. The method of claim 8,further comprising: determining a percentage of at least one ofAromatics, Olefins, Saturates in a sample.
 19. The method of claim 8,further comprising: determining a percentage of contamination from anequation developed by correlation to a training set.
 20. The method ofclaim 8, further comprising: Determining a percentage of CO₂ in naturalgas in a sample. Improved correlations for physical propertiescomprising density, viscosity, pressure, volume, and temperatureproperties.
 21. The method of claim 8, further comprising: Determining apercentage of contamination using high-resolution spectra downhole tomonitor sample cleanup.
 22. The method of claim 8, further comprising:Determining a percentage of aromatics to monitor sample cleanup.
 23. Amethod of measuring the spectrum of a fluid sample in a downholeenvironment comprising: lowering a sonde containing a samplingspectrometer into a bore hole; illuminating a fluid sample in thesampling spectrometer; directing light from at least one interferencefilter onto the fluid sample; rotating the one or more interferencefilters to obtain different wavelengths of light transmitted through thefilters as the angle of incidence of the light upon the rotating filterchanges; recording the intensities corresponding to those wavelengths;and correlating angular position of the filter with measured lightintensity to obtain a measure of the spectrum of the fluid sample atdifferent wave lengths of light.
 24. A downhole spectrometer comprising:A sonde for traversing a borehole; An instrument body in the sondecomprising: At least one interference filter attached to a spindle sothat, upon rotation, the angle at which light strikes the filterchanges; and at least one light-detecting devices, with eachlight-detecting devices paired with one interference filter; acollimated light beam incident on at least one interference filter, aportion of light incident upon the interference filter being transmittedthrough the sample, the transmitted light incident upon thelight-detecting devices with which it is paired; and A rotator componentto rotate the angle of the spindle and its attached interference filtersrelative to the incident light upon the interference filters.